My father left the family when I was six after he had an affair. We didn't hear from him for 20 years.

My mum had depression and was left in huge debt as my father refused to contribute financially – although she never said a bad word about him. I have always felt some sadness and longing for a father figure and felt guilty for leaving my mum whenever I went out and when I left home. None of this came from my mum; she always encouraged us to live our own lives.

Three years ago my paternal grandmother died. We always knew that this would be the time when we were likely to see my father again. When we met, he was timid but made it clear he regretted the past and wanted to start some kind of relationship with us. Although hesitant, I tried to be open-minded and felt that, in time, this could happen and that a part of me still wanted a relationship with him.

Nine months later my mum told me and my brother that she had started a relationship with my father. He was still married and living with my stepmother; the affair went on for more than a year before he summoned up the courage to leave her. He now spends alternate weekends with my mum and has basically picked up where he left off – they see all their old friends and live in the same town.

My relationship with my mum has deteriorated. I am angry that she would have an affair and do what my stepmother did to her; angry that she would choose to go back to a man who caused so much upset, and so sad that I cannot be a part of her life as I used to be. However, when we do see or speak to each other (less than once a month now), she acts as if nothing is wrong.

Part of me thinks that I should just get on with it and at least make the move to try to build a relationship with him, but I can't get over the hurt that he has caused. At the same time, I feel I am being unreasonable and worry that I am being childish. I also feel immense pressure to do something but don't know what.

A, via email

It is hard to tell if you feel more let down by your mother or your father. It sounds as if you could have rebuilt a relationship with your father, but then he had an affair with your mother. You have invested huge amounts in your mum. Despite you saying she has encouraged you to live your own life, you have suffered guilt; maybe you have stayed home when you wanted to be out. You propped her up, felt responsible for her. Part of you probably admired her for being stoical, keeping the family together, never bitching about your dad. But now it is almost as if she has been disloyal to you and chosen the cheating, lying dad over you. She hasn't: she is still your mum.

Neither of your parents have been the people you wanted them to be at various times in your life and that can't be easy. But I learned something a few years ago that I found liberating, which is that you cannot control anyone else's behaviour, only your own. Once you understand and accept this, it will concentrate your mind on what you want, how you want to behave, what you want to do.

There was a line in your original, longer letter that jumped out at me – that you are scared of upsetting your dad. I wish you had explained more of what you meant by this.

Maria Olteanu, psychotherapist, (www.ukcp.org.uk), thinks: "You, and the whole family, have been 'on hold' [and I would say slightly controlled by your dad, even in his absence] since you were six. And now it is almost as if you are travelling back in time."

Your parents are back together, same friends, same town. But so much has happened in between. "Things have not evolved in one established and logical direction, but they went forwards and backwards," suggests Olteanu, which "has generated pain and high emotional confusion".

So, what to do? Olteanu suggests letting go of things that are not yours to understand and says you seem confused as to whom you should love/trust. Therapy would really help you with this. In the meantime, you don't need to do anything about your parents. Invest in yourself for now: your friends, life, work, relationships. Anchor yourself a bit so that your emotional wellbeing is not totally tied to your parents. Once you feel stronger, build on your relationship with your mother. I think you want to, but feel let down by her. I bet she misses you. As for your dad, he really has some work to do, but that is up to him. Don't sabotage his efforts, but don't let him influence who you spend time with. For a weak man, you are all giving him far too much power.